A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, one or more parameters of the patterned substrate are typically measured, for example the overlay error between successive layers formed in or on the substrate. There are various techniques for making measurements of the microscopic structures formed in a lithographic process, including the use of a scanning electron microscope and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and one or more properties of the scattered or reflected beam are measured. By comparing one or more properties of the beam before and after it has been reflected or scattered by the substrate, one or more properties of the substrate may be determined. This may be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with a known substrate property. Two main types of scatterometer are known. A spectroscopic scatterometer directs a broadband radiation beam onto the substrate and measures the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. An angularly resolved scatterometer uses a monochromatic radiation beam and measures the intensity of the scattered radiation as a function of angle. An ellipsometer measures polarization state.
In order that the radiation that impinges on to the substrate is diffracted, an object with a specific shape is printed on to the substrate and is often known as a scatterometry target. The object may be a diffraction grating or the like which is made up of an array of bars or other periodic structures having a cross-section which is known as the profile. Indeed, the profile is typically a repeating set of similar structures such as resist lines. The average profile of one such repeating structure (or unit cell) is called “the profile”. The actual profile of the object is hence a concatenation of a number of unit cells, which may contain local variations.
The profile is generally measured from the surface of the substrate and may include one or more product layers on which an object is deposited. Ideally, the object that is printed on to the substrate would have a predetermined shape and would be printed perfectly each time it was printed. In practice, however, the printed shape and size of the object may deviate from the ideal shape, predominantly because of the difficulty in creating accurate shapes at the small size of the object involved. It is desirable to have a system to determine how exactly the object is shaped.
As mentioned above, it is possible to determine the actual shape of a scatterometry object using a cross-section scanning electron microscope and the like. However, this involves a large amount of time, effort and specialized apparatus and is less suited for measurements in a production environment because a separate specialized apparatus is required in line with normal apparatus in, for example, a lithographic cell.
Another way to determine the profile of a scatterometry object is to diffract a beam of radiation from the object and compare the diffraction pattern with model diffraction patterns that are stored in a library of diffraction patterns alongside the model profiles that create these model patterns.
In addition to using a library of model diffraction patterns, there are further methods that either do not use a library or that are used in combination with a library. One such method is an iterative method, where the parameters are given a starting value and the diffraction pattern of these starting values is calculated and compared with the measured diffraction pattern. The values of the parameters are then iteratively changed to improve the match between the iteratively modeled and measured diffraction patterns.